1. Field of the Invention:
This invention relates to an image forming device utilizing electrophotographic process such as an electrostatic copying machine, a printer, etc. and a transfer belt to be used for these image forming devices.
2. Related Background Art:
There has been already proposed an image forming device which is constituted such that a transfer material is electrostatically adsorbed and conveyed on the surface dielectric layer of a transfer drum, etc., brought along to the transfer site where an image carrying member exists and the toner image formed on the image carrying member by the electrical field from a transfer belt, etc. is transferred onto the transfer material, as can be seen in U.S. Pat. Nos. 2,576,882 and 3,244,083.
Such a device is constituted so as to transfer the toner image by generating a transfer electrical field by discharging by means of a corona discharger from the back of the insulating transfer belt, and also is provided with a corona discharger for having the transfer material electrostatically adsorbed onto the transfer belt prior to the transfer charging.
The device which performs electrostatic transfer by having the transfer material electrostatically adsorbed onto the surface of a dielectric material layer such as the above device sometimes suffers from defective adsorption of the transfer material defective transfer due to accumulation of charges of the opposite polarity to the transfer electrical field on the surface of the dielectric material is caused peeling discharging during separation of the transfer material from the image carrying member after transfer and thereafter during separation of the transfer material from the transfer belt.
In order to avoid such situation, as disclosed in Japanese Patent Publication No. 61-40990, there have been known the method in which the potential in which the dielectric material surface is stabilized by effecting alternating current discharging onto the dielectric material surface. Another method is known in which the surface potential is stabilized by performing corona discharging with alternate current or corona discharging with direct current of different polarities from each other from the surface and the back of the dielectric material in a multiple transfer system such as multi-corona copying, etc.
However, such methods not only generate a large amount of corona products by corona discharging, particularly ozone to deteriorate the surface of the dielectric material and also have deleterious influences on the image carrying member. Also corona discharging onto a dielectric material is poor in potential stability, extremely unstable and depending highly on the environment because of having no standard potential surface, particularly when the transfer belt, etc. consist only of a dielectric material.
For prevention of such troubles by corona discharging, for example, Japanese Patent Application Laid-open No. 56-15477 discloses a technique in which a transfer belt surface in contact with the transfer material is formed on an insulating layer of 10.sup.10 .OMEGA. cm or higher. Moreover, the back is made of an electroconductive layer of 10.sup.8 .OMEGA. cm or lower, which electroconductive layer is grounded to contact charge the insulating layer.
This one is excellent in potential stability of the dielectric surface, since the electroconductive layer acts as the standard potential surface to effect contact charging.
However, in this case, since the electrical field intensity of contact charging is high, and therefore if there is any defect such as a pinhole in the insulating layer, high voltage leakage will occur. Moreover for example, when an electroconductive brush is used as the charging means, defective transfer will occur due to shortage in charging over that area.
Particularly, in the case of the transfer belt as decribed above, transfer efficiency is better as the insulating layer is thinner. Accordingly the thickness of said layer should preferably be 100 .mu.m or less, but in such thin layer, it is extremely difficult to produce a layer without the pinhole. Since the transfer electrical field is formed only with surface charging in the constitution as described above, when the transfer material is lowered in resistance under high temperature and high humidity temperature, defective transfer, defective adsorption due to charge leakage onto the tranfer belt surface are liable to occur.
Further, as described above, due to uniform electrical field on the transfer belt surface, even at the position before the position where the image carrying member is in contact with the transfer belt, the action of attracting the toner of the image carrying member surface onto the transfer belt is exerted, whereby the toner will fly toward the transfer belt to effect unreliable transfer, but the so called "scatting" phenomenon occurs to bring about deterioration in image quality. If the electrical field intensity is weakened for prevention of this, there will be generated the "inner drop-out" phenomenon in which only the peripheral portion of linear image is transferred.
As the transfer belt, there have been known one shaped in a belt by seaming dielectric films such as polyethyleneterephthalate film, etc. by such means as thermal fusion, ultrasonic fusion, etc., and one having an electroconductive member such as aluminum, gold, tin oxide, carbon, etc. vapor deposited in the inner layer of the above dielectric film, one having the above film wound on a rubber such as NBR, SBR, EPDM, etc. which is made electroconductive.
As the endless belt, as shown in Japanese Patent Application Laid-open No. 62-156682, it has been disclosed to form an endless belt by providing a thermoplastic resin layer with a volume resistivity of 10.sup.14 .OMEGA. cm, or higher and a thermoplastic elastomer or ionomer of 10.sup.6 .OMEGA. cm or lower as its inner layer and extrusion blow molding both layers interally.
Further, Japanese Patent Application Laid-open No. 62-203169 discloses and endless belt, having a surface dielectric material layer of a thermoplastic elastomer or ionomer having a volume resistivity of 10.sup.12 to 10.sup.16 .OMEGA. cm and its inner layer of a semiconductive thermoplastic elastomer or ionomer having a volume resistivity of 10.sup.7 to 10.sup.12 .OMEGA. cm, both being integrated by extrusion blow molding.
However, the following problems are involved in the transfer belt of the above prior art. For example, in a system having a dielectric material film such as polyethylene-terephthalate thermally fused or sonication fused, due to the step difference at the seamed portion, the toner portion attached on the transfer belt is pooled there and cannot be completely cleaned to cause back staining of the transfer material, and further the problem occurs in durability because of weak strength of seam.
Also, in an insulating dielectric material film, it is difficult to stabilize the dielectric material surface potential. When the transfer material is peeled off from the electrostatic latent image carrier, charges of the opposite polarity to the transfer electrical field are accumulated on the dielectric material surface by peeling discharging during separation of the transfer material from the transfer belt, whereby the charge-up phenomenon may sometimes occur to give rise to defective adsorption of the transfer material, defective transfer of toner image.
On the other hand, as indicated in Japanese Patent Application Laid-Open Nos. 56-154772 and 56-150362, the constitution having an electroconductive layer provided on the back of the dielectric material film has been known to maintain constantly the surface layer by contact charging, corona charging, etc. with the electroconductive layer as the standard potential surface. In this system, the problem of the seam of the dielectric material film is the same as in the above prior art example, but the charge-up phenomenon hardly occurs. However, since it has an electrocoductive layer on the back, the electrical field on the transfer belt surface is uniform, and therefore the electrical field acts in the direction of attracting the toner on the elecrostatic latent image carrier toward the electrostatic latent image carrier before the contact between the image carrier and the transfer belt, whereby said toner flies away toward the transfer belt to effect an unreliable transfer of the toner image. Thus gives rise readily to the phenomenon of so called "scattering" (the state in which toner particles are scattered around the letter portion). If the electrical field on the transfer belt surface is weakened for preventing this phenomenon, there are such disadvantages that the so called "inner drop-out" phenomenon may occur in which only the peripheral portion of the letter portion is transferred, and that lowering in transfer efficiency is bought about.
In Japanese Patent Application Laid-open No. 62-156682, an endless belt having an electroconductive layer as the inner layer is disclosed, and since the belt is endless, the problem of the step difference of seam, etc. can be cancelled, but the drawbacks caused by having the electroconductive layer as the inner layer cannot be cancelled.
On the other hand, in the endless belt for transfer disclosed in Japanese Application Laid-open No. 62-203169, both the surface layer and the inner layer are formed of thermoplastic elastomer layers, and therefore there are posed problems of abrasion, deformation, elongation, etc. by prolonged successive uses, and in further worse case, there is also a problem that the transfer belt may be cut during successive uses. Also, the surface frictional resistance is great and cleaning characteristic of the transfer belt is inferior.